Conventionally, a large hollow structure, i.e. a structure having a diameter and/or a length which may be as much as several meters, such as a container, comprises several sub-parts welded together, namely, in general, a main part forming the central body of the container and two end parts each fastened to one end of the said container body by welding.
In other words, in the case of a container, the two ends of the container are welded to the body of the container by implementing a welding process, for example an automatic plasma welding process, in which the welding torch is located outside the container and the container itself being rotated about its axis, so as to produce a uniform weld around the entire periphery or circumference of the said container, all along the mating plane defined by the junction of the two sub-parts to be welded to each other.
Alternatively, it is also possible to keep the parts to be welded stationary and to move the welding torch or torches around the parts to be welded.
In the case of the manufacture of metal tubes, the sub-parts of a tube are in fact, in the context of the present invention, the two opposed longitudinal edges of the same metal sheet or plate, these edges being brought into contact with each other (forming a "U" and then an "O ") before being welded together into a "gastight" tube.
In other words, in the context of the invention, the sub-parts may belong to the same piece (in the case of a tube) or to separate structures (in the case of a container) which have to be brought together, one into contact with the other.
In practice, it has been observed that, during the welding operation, that part of the weld joint located on the opposite side from the weld, i.e. on the inside of the volume defined by the container or the tube, is very sensitive to oxidation when the metal is still molten or at high temperature.
It is therefore necessary, throughout the welding operation, to shield the weld joint while it is being produced by providing a gas shield not only on the outside but also on the inside of the tube or of the said container.
To do this, the air inside the container or the tube must be purged and replaced by an inert or neutral atmosphere which is more or less free of oxygen.
For this purpose, it is common practice to use argon or nitrogen coming from a cryogenic production source used for producing cryogenic fluids, especially nitrogen and argon, with a purity conventionally greater than 99.9%, for example a cryogenic distillation unit.
After the nitrogen or argon has been produced by cryogenic distillation, it is held in storage and transportation containers, such as gas bottles, and then subsequently taken to the site where it will be used by a suitable transportation means, such as a lorry.
However, using nitrogen or argon produced by cryogenic distillation and then taken to the site where it will be used, i.e. the welding site, has a certain number of drawbacks, especially because it requires:
strict monitoring of what is in stock and what is being delivered, in order to avoid any lack of product on the site of use; PA1 tight control of the production and delivery costs in order to minimize their impact on the final cost of the end-product, i.e. the welded tube or container; and PA1 a site for storing the gas bottles not yet used. PA1 at least the first and second sub-parts are brought into contact with each other so as to form an unwelded hollow structure defining an internal volume; PA1 at least part of the said internal volume of the said hollow structure is at least partially inerted by introducing an inerting gas mixture into the said hollow structure, the said inerting gas mixture comprising from 93% to 99.8% nitrogen and from 0.2% to 7% oxygen; and PA1 the said first and second sub-parts are welded together so as to form a welded hollow structure, for example a container or a tube. PA1 the inerting gas mixture comprises from 95% to 99.7% nitrogen and from 0.3% to 5% oxygen and preferably the inerting gas mixture comprises from 97.5% to 99.6% nitrogen and from 0.4% to 2.5% oxygen, preferably about 2% oxygen and the balance being essentially nitrogen; PA1 the inerting gas mixture furthermore comprises gaseous impurities chosen from argon, carbon dioxide, carbon monoxide and water vapour, the total content of the said impurities preferably being less than 0.5%; PA1 the inerting gas mixture is produced from air separated by permeation, the inerting gas mixture preferably being produced on site and introduced into the internal volume of the said hollow structure; PA1 the separation of the air by permeation is carried out by means of at least one membrane or of at least one permeation membrane module and the inerting gas mixture is recovered on the rententate side of at least the said membrane or of the said membrane module; PA1 before starting to inert the internal volume of the hollow structure, the internal volume of the hollow structure is substantially hermetically sealed; PA1 the inerting operation is carried out at least before the start of welding by gradually replacing the ambient air contained in the hollow structure with the inerting gas mixture, the inerting operation preferably being continued throughout the duration of the welding; PA1 the inerting gas mixture is introduced into the hollow structure at a pressure of between 5.times.10.sup.5 Pa and 10.sup.6 Pa, preferably of the order of 8.5.times.10.sup.5 Pa approximately; PA1 throughout the duration of the welding of the first and second sub-parts to each other, a pressure of less than 5.times.10.sup.5 Pa, preferably between 2.times.10.sup.5 Pa and 3.times.10.sup.5 Pa approximately, is maintained inside the hollow structure; PA1 the arc welding is plasma welding or TIG welding or a TIG-plasma combination; PA1 the first and second sub-parts are sub-parts of a container, preferably the first sub-part forms at least one part of the central shell of the container and the second sub-part is an end element of the container; PA1 the first and second sub-parts form a metal tube, the first and second sub-parts forming the longitudinal edges of a single metal plate or sheet, which edges are brought into contact with other (forming a "U" and then an "O") and then welded together so as to obtain a hollow metal tube. PA1 holding means allowing at least the said first and second sub-parts to be brought into contact with each other and held together so as to form an unwelded hollow structure defining an internal volume; PA1 inerting means allowing at least part of the said internal volume of the said hollow structure to be at least partially inerted by introducing an inerting gas mixture into the said hollow structure, the said inerting means including: PA1 permeation means comprising one or more permeation membranes or membrane modules producing an inerting gas mixture comprising from 90 to 99.8% nitrogen and from 0.2 to 10% oxygen from air; PA1 inerting-gas supply means allowing the said inerting gas mixture to be introduced into the said hollow structure; and PA1 welding means including at least one arc-welding torch, preferably of the plasma and/or TIG type, allowing at least the first and second sub-parts to be welded together so as to form a welded hollow structure.